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David Corwin
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Conjugacy Classes as Left Kan Extension of Forgetful Functor

Let $\mathbf{Set}$, $\mathbf{Grp}$, and $\mathbf{Grp}^{\rm conj}$ denote the categories of sets and functions, groups and homomorphisms, and groups and homomorphisms up to conjugation, respectively. (Thus the fundamental group is a functor from path-connected spaces to the latter.)

Let $X$ be the forgetful functor from $\mathbf{Grp}$ to $\mathbf{Set}$, $A$ the obvious functor from $\mathbf{Grp}$ to $\mathbf{Grp}^{\rm conj}$, and $L$ the functor from $\mathbf{Grp}^{\rm conj}$ to $\mathbf{Set}$ sending a group to its set of conjugacy classes. Then is $L$ the left Kan extension of $X$ along $A$?

David Corwin
  • 15.4k
  • 10
  • 83
  • 123